The following will describe conventional digital data demodulators and digital data modulators by taking conventional digital data modulator-demodulators as an example. The conventional digital data modulator-demodulators are used for Compact Disc (CD) recorders/players.
In conventional CD recorders/players (conventional digital data modulator-demodulators), when information to be recorded is recorded on a CD, additional information such as an error-correcting code and a data attribute is first added to first digital data, which is information to be recorded on the CD, to constitute second digital data. Subsequently, Eight to Fourteen Modulation (EFM) is performed on the second digital data to constitute third digital data satisfying RLL [2, 10] (abbreviation of Run Length Limited, the minimum run length is 2 and the maximum run length is 10). Further, a synchronous pattern is disposed at the front of modulation data for each predetermined number of modulation data, and a 3-bit merge bit is added to the front of a synchronous pattern and the front of the modulation data.
EFM is processing for converting an 8-bit length data symbol, which constitutes the second digital data, into modulation data of 14 bits.
A data block configured thus is referred to as a frame (modulation data block), in which the synchronous pattern is disposed at the front of modulation data for each predetermined number of modulation data and a merge bit string is disposed at the front of the synchronous pattern and the front of the modulation data.
Thereafter, NRZi (Non Return to Zero Inverted) conversion is performed on the third digital data to enable pit formation according to a run length. Then, pits and space are formed on the data recording surface of the CD according to data obtained after the NRZi conversion.
As described above, in the conventional CD recorder/players (conventional digital data modulator-demodulators), the first digital data is recorded on the CD by forming pits on the CD.
For example, Japanese Patent Laid-Open No. 2000-242929 discloses a digital data modulator-demodulator, in which when additional information different from first digital data is recorded on a CD, the additional information is converted into a deviation from the track center, and a pit is formed on the CD according to the deviation (See Japanese Patent Laid-Open No. 2000-242929, pp. 7-8, 11-12, FIGS. 2 and 8).
As described in Japanese Patent Laid-Open No. 2000-242929, the conventional recording method is also applicable to a digital versatile disc (DVD) (See Japanese Patent Laid-Open No. 2000-242929, pp. 11-12).
For example, as described in Japanese Patent Laid-Open No. 2000-242929, in the conventional CD recorders/players (conventional digital data modulator-demodulators), when information recorded on a CD is reproduced, fourth digital data corresponding to third digital data after NRZi conversion is first reproduced from a pit formed on the CD. Subsequently, NRZ conversion is performed on the fourth digital data to reproduce fifth digital data corresponding to the third digital data. Then, EFM demodulation is performed on the fifth digital data to reproduce sixth digital data corresponding to the second digital data. After that, error correction is performed on the sixth digital data according to a synchronous pattern, so that the first digital data recorded on the CD is reproduced (See Japanese Patent Laid-Open No. 2000-242929, pp. 8-9, FIG. 3).
Moreover, for example, Japanese Patent Laid-Open No. 7-244935 discloses a conventional digital data modulator-demodulator which improves the detection and protection of a synchronous pattern included in fifth digital data in order to improve the capability of reproducing sixth digital data (See Japanese Patent Laid-Open No. 7-244935, pp. 4-5, FIGS. 1, 2, 3).
However, in the conventional digital data demodulators, a modulation code (a data string obtained by adding a merge bit string of 3 channel bits to modulation data of 14 channel bits) cannot be specified in some states of an error occurring in a transmission line, and thus digital data which can be modulated may be lost before a synchronous pattern is specified.
Further, in pit formation defined as a transmission line (meaning recording on an optical disc), when additional information is recorded by forming a pit deviated from the recording track center of the optical disc, the following problem occurs: when a pit deviated from the recording track center is formed, reflected light for reading the pit is affected by a pit formed on an adjacent track and an uncorrectable error occurs in the transmission line. Thus, in the conventional digital data demodulators, digital data which can be modulated may be lost before the subsequent synchronous pattern is specified.
When the detection and protection of a synchronous pattern is improved, although a synchronous pattern is more likely to be specified, digital data which can be modulated may be lost before the synchronous pattern is specified.